字幕表 動画を再生する 英語字幕をプリント The future is going to require us to make some dramatic changes to the way we produce and consume electrical power. Climate change is happening whether we like it or not, and switching to cleaner sources of energy is the main way we'll be able to keep it from getting worse. And we'll run out of fossil fuels eventually anyway. That's why engineers are looking for ways to scale some of the existing sources of energy that don't rely on fossil fuels, and maybe even find sources that are entirely new. There are all kinds of promising candidates – from nuclear fusion to plain old hydrogen to …poop. [Theme Music] For at least the last 450 million years, another form of life has been harvesting all it needs from the sun without doing any harm to the planet. I'm talking about plants. The chlorophyll in a plant's cells absorb light and use its energy to combine carbon dioxide and water to produce glucose and oxygen. That's photosynthesis. Eliminating some carbon dioxide from the atmosphere and producing the oxygen we breathe is already pretty great, but there's another useful aspect to this. When the plants store some of that glucose, it's also storing chemical energy. And where there's energy, there's an engineer who will find a way to use it. Biomass energy is a form of energy that relies on burning biological matter, like plants, either directly or by processing them into a fuel. As it burns, the chemical energy stored in the plant's matter is converted into heat. Like with many of the other energy sources we've mentioned, the heat turns water into steam, which in turn does work on a turbine; in other words, it drives a heat engine. A generator then converts the turbine's motion into electrical power. About half of all biomass energy is delivered by burning plant matter directly, which humans have been doing long before the need for electrical power. A wood burning stove can keep your house warm in the winter, or it can be used as a heat source for something else – to cook pizza, for example. On a larger scale, scrap wood left over from wood processing or from waste produced by humans, along with food waste, can be burned in power plants more directly. Most of the other half of biomass energy comes from processing biological matter into biogas or biofuel. By pulping and chemically treating biomass derived from crops and other plant matter, it becomes easier for special proteins called enzymes to chemically break it down into a more readily usable fuel source. One example of this is our old friend ethanol – the type of alcohol you'll also find in drinks. Ethanol is produced from enzymes breaking down crops such as wheat or corn. That can then be turned into a liquid fuel suitable for burning. But it's not just plants that we extract energy from! As certain types of agricultural waste, like manure, decay, they can release what's known as biogas. These are gases like methane that can be burned as a source of fuel. There's also another small contributor to biogas production: human waste. When I said engineers will find a way to get energy from everything, I meant everything. Biofuels are already being used pretty widely. In Brazil, 4 out of every 5 cars produced have hybrid engines capable of burning both ordinary gasoline and bio-ethanol. To be clear, burning biomass fuels does release carbon dioxide into the atmosphere, just like fossil fuels. But, because a nearly equivalent amount of CO2 is captured from the atmosphere during photosynthesis to store chemical energy, on the balance of things, biomass energy is what's known as carbon neutral. We can also always grow more plants, which makes biomass more renewable than fossil fuels. But given that plants absorb CO2 as they grow, you could argue that we shouldn't just release it all back by burning them. And processing biomass into biofuels often requires some amount of energy input, which indirectly releases more CO2. Humans already use about a third of all the CO2-absorbing plant matter on Earth. Destroying even more of those plants to release energy could be catastrophic for certain ecosystems. And even though we could grow more plants, we'd have to trade off the use of land, water, and chemical resources between growing food or providing energy. Demand for all of those resources is already high, and projected to continue growing pretty quickly. As with any other power system, there are places where engineers can make better of use of what's already there, like improving the design of those hybrid engines that burn both ordinary gasoline and biofuels. We also might be able to use different biomass sources, like algae, or find more efficient chemical reactions for processing biomass into fuel. That might unlock new, more sustainable ways to use biomass for power. But there are other futuristic power sources that don't release any CO2 while being consumed. Although as we'll see, that doesn't mean they're carbon neutral! Hydrogen is the most abundant element in the universe, which makes it pretty surprising that pure hydrogen is very rarely found on Earth. It consists of a single proton and electron, and if you can produce it, it's the perfect source of fuel for the very aptly named hydrogen fuel cell. Fuel cells of this kind use a chemical reaction between hydrogen and oxygen to directly generate electrical power. And the only other byproduct of this reaction is water. Much better than carbon dioxide. There's a fairly big problem, though. If hydrogen isn't naturally produced on Earth, how do you get it? You can use the process known as electrolysis to chemically separate water into hydrogen and oxygen, and just store the hydrogen. Except, performing electrolysis uses energy. In fact, producing hydrogen fuel requires more energy to produce than you get from using it in a fuel cell. Efficiency-wise, that's not great. But there are good arguments for having hydrogen fuel cells in your power-producing arsenal. For starters, because hydrogen is the lightest element, hydrogen fuel is incredibly lightweight, which makes it great for transport, like on spacecraft. And unlike solar-powered devices, if you have hydrogen fuel at the ready, fuel cells don't require recharging like batteries. So they're great for indoor vehicles like forklifts, where releasing lots of fumes could be problematic, but you also need to use them pretty much continuously, which makes batteries less practical. Finally, if we can efficiently produce hydrogen fuel by electrolysis from the surplus energy provided by solar power on especially sunny days, hydrogen fuel could give us a carbon neutral way of storing electricity. On a larger scale, there is a power producing process you've already heard of that's already used to provide a good deal of energy – 10% in the US – while releasing comparatively tiny amounts of CO2: nuclear fission. In fission, an atom splits in two, releasing a lot of energy in the process. Nuclear power releases the same amount or even less of those greenhouse gases than most renewable energy sources. But don't be fooled, it's a non-renewable source of energy! The main fuel used in nuclear fission, uranium 2-3-5, is a limited resource that has to be mined and purified from the ground, much like fossil fuels. The way we get power from uranium is by assembling rods of uranium parallel to one another and setting up a chain reaction. The nucleus, or core, of a uranium atom is made up of protons and neutrons. If a fast moving neutron hits a uranium atom at just the right energy, it can split the uranium in two. The uranium splits into atoms of other elements, like krypton and barium. But three of the neutrons from the nucleus will fly off, carrying some energy with them. Those neutrons can then collide with another uranium atom, causing fission that releases even more neutrons, and so on. Meanwhile, the cascade of splitting atoms gives off gamma radiation and heat, heating up the reactor. So fission turns a nuclear reactor into a heat source for a power plant. Unfortunately, once you've used up all the useful uranium in the rods, you're left over with the biggest setback of nuclear power: nuclear waste. Nuclear waste consists of radioactive material that emits highly energetic particles that can be extremely dangerous for any living thing, including humans. Dealing with it safely is the sort of issue nuclear engineers can help with. They also design nuclear power plants to carefully control fission to stop it turning into a runaway process. If that happens, it can lead to disasters like the kind that happened in Chernobyl or Fukushima. As for nuclear waste, nuclear engineers aim to find as safe a way as possible for disposing of it. Most of the time, used-up uranium rods are put into thick steel containers and buried in deep underground vaults far from any people, or kept in tanks near the nuclear power plants themselves. Neither of these solutions is ideal, and engineers may find a better way of handling the problem in the future. But it would be better if we could find a way of generating nuclear power that produces no radioactive fuel as waste at all. Which is where nuclear fusion comes in. It's the same energy-releasing process that occurs in the sun. So naturally, getting it to happen here on Earth is something many engineers are looking into! In fact, the National Academy of Engineers in the US has declared the goal of providing energy from fusion one of its grand challenges for the 21st Century, in addition to the improved solar power we talked about last time. The major setbacks to providing energy from fusion are the intense amounts of heat and pressure that atoms need to fuse. Without the gravitational strength of the sun on hand, nuclear physicists and engineers have to design some of the world's most powerful magnets to contain plasma: an extremely hot gas made up of ions, or atoms with an electric charge. Unfortunately, the magnets require energy to operate, so fusion has to deliver more power than the magnets consume for it to be useful. And we haven't figured out how to do that yet. In 2018, a test reactor in the U.K. announced they'd reached temperatures of 15 million °C in a plasma – well on the way to a sustainable fusion reaction. And currently under construction in the South of France is what will be the world's biggest fusion plant, called ITER. The design is still experimental, but the hope is that it will be capable of delivering the first ever self-sustaining fusion process capable of generating more power than the magnets consume. Engineers have already contributed to the effort by designing more efficient magnets and contributing to the design of ITER itself. But if fusion ends up being a suitable power source, they'll have a lot more work left to do to scale it for wider use. Between the sources like solar and wind power we talked about last time, and less-developed tech like nuclear fusion, there are lots of different ways we can change the future of energy for a world less reliant on fossil fuels. No matter what, we'll need a new power infrastructure to support the cleaner energy world of the future. And that infrastructure, that future, will be built by engineers. In this episode we looked at alternative energy sources. We saw how biomass can be burned as a fuel source, how hydrogen can be used in a fuel cell to generate electrical power, and how nuclear fission provides power to the grid. Finally, we saw how nuclear fusion might someday do the same without any radioactive waste. Next time, we'll be looking at ways of storing all that power when we look at energy storage and batteries. Check out our new Augumented Reality Poster, available now at dftba.com! Crash Course Engineering is produced in association with PBS Digital Studios. Check out our sister channel Physics Girl, in which Dianna Cowern explains the physics behind puzzling phenomenon and everyday mysteries. Subscribe at the link in the description. Crash Course is a Complexly production and this episode was filmed in the Doctor Cheryl C. Kinney Studio with the help of these wonderful people. And our amazing graphics team is Thought Cafe.